[Bf-docboard] Inverse Coefficients falloff mode documentation patch

[Jack Andersen]

I've written a preliminary patch for the 'Inverse Coefficients'
falloff mode available in 2.78-rc1. If someone is able to review, it'd
be much appreciated!

I'm also wondering if anyone has access to the sphere-grid .blend used
to illustrate falloff modes? It'd be useful to illustrate a
quadratic/constant mix.

https://developer.blender.org/D2241